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Lecture FOUR
Metamorphic Reactions and
Protoliths of Metamorphic
Rocks
Development of Metamorphic Rocks
The yielded metamorphic rocks, with specific
mineral composition and textures is a function of:
 Protolith nature i.e: whole rock chemistry
(pelitic
(Argillaceous),
semipelitic,
calcareous
(limestone and dolomite), mafic-ultramafic, basic
igneous,
granitic,
Mn-rich
sediments,
ironstone,
laterites... etc.)
 P-T-X conditions (the intensity of temperature and
the
intensity
and
type
of
pressure
(simple
compressed or twisted and broken) influence and the
presence or absence of fluids and their chemistry
during metamorphism)

Time (who long the rock subjected to HT and
HP?. By which the grain size was evolved, and the
reactions were proceed)
Metamorphic Rocks components and development
A metamorphic rock consists of individual grains of several solid
minerals and a network of grain boundaries, which at the time of
metamorphism may have held an aqueous fluid, providing pathways for
transport through the rock.
garnet
garnet
Omphacite
Development of Metamorphic minerals
For a new mineral to appear by a chemical reaction, a number of
processes have to operate in concern:
 Nucleation: nuclei (embryo crystals) of
the new mineral appear
 Interface reactions - dissolution:
reactant minerals break down, their chemical
constituents going into solution
 Interface reactions - growth: material
is added onto the nuclei to build larger
crystals
 Mass transfer: material is transported
through the rock from sites of breakdown to
sites of growth
Nucleation, Mineral growth and Grain size
-Completed reaction produces an amount
of product (mineral phases). The
microstructure, will depend on the
relative rates of nucleation and growth of
minerals
- Grain size in a metamorphic rocks is a
function in:
- Intensity of P-T conditions,
- nuclation rate, and
-Time interval of metamorphism
-Coarse-grained rocks are the product of
long sustained metamorphic conditions
(possibly over millions of years) at HT
and HP (e.g. in high grade regional
metamorphic rocks)
-Fine-grained rocks are products of LP,
LT, in some cases, short reaction time
(e.g. in contact metamorphiic rocks)
Metamorphic Reactions and P-T path
- With increasing P-T conditions, metamorphic reaction toke place (e.g.
burial effect) until the maximum pressure and temperature (peak
condition), then with decreasing the P-T conditions (e.g. uplift) until
cooling of the rock. This is known as Metamorphic P-T path
- The P-T path include three
segments:
 Prograde segment: With
increasing the P-T conditions
(such as burial effect)
 Peak segment: at maximum PT conditions (at the summit
metamorphic conditions)
 Retrograde segment: With
decreasing P-T conditions (such
as uplift)
- The metamorphic P-T can be simple (clockwise or anticlockwise) or
complex due to multiphase metamorphism
Types of Metamorphic reactions
- With
either progressing or retrogressing metamorphism,
various types of metamorphic reactions are proceeds e.g.:
1- Univarient reactions: reactions that plot as line or
curve on the P-T diagram and depend on temperature and
pressure only e.g:
Al2Si4O10(OH)2  Al2SiO5 + 3 SiO2 + H2O
Pyrophyllite
Al-silicate + Qtz + fluid
Cont. Types of Metamorphic reactions
2- Divarient reactions: reactions
occur over wide range of P-T. This
because most minerals involved
in
the
solution
reaction
(e.g
plagioclase);
exhibit
garnet,
therefore,
solid
mica,
the
reaction boundaries can changed
depend on the composition of
solid solution.
KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
Ms
Qtz
Kfs
Sill
W
Cont. Types of Metamorphic reactions
3- Solid-soild reaction: only
involve the solid-phases for both
reactant and products (with no
fluid phases). So reactions
involves phase transformation
e.g.
Calcite  aragonite,
andalusite  sillimanite,
graphite  diamond
Albite  jadite + quartz
Cont. Types of Metamorphic reactions
4- Dehydration reactions: reactions
that liberate H2O. This always occur in
the prograde reaction, i.e. with
increasing temperature e.g
chlorite + muscovite 
orthoclase + andalusite + H2O
5- Decabonation reaction : reactions
that liberate CO2 e.g
Calcite + quartz  Wallstonite + CO2 
Cont. Types of Metamorphic reactions
6- Oxidation-reduction reaction: reactions that involve change the
valence state of Fe-Ti oxide phases (Fe+2 and Fe+3) e.g: the
breakdown of biotite to K-feldspars and magnetite at high P-T
cinditions
biotite + O2  K-feldspars + Magnetite + H2O
7- Cation exchange reaction: reaction involves ionic substitution
of two or more phases in the system e.g:
Fe-garnet + Mg-biotite  Mg-biotite + Fe-garnet
8- Ionic reactions: reaction that balanced by inferring involvement
of ionic species derived from the fluid phase
Protoliths of metamorphic rocks
As we discuss, The yielded metamorphic rocks is function
of:
- Protolith (original rock) nature  bulk-rock chemistry
- P (pressure)-T (temperature)-X (active fluids) conditions
- Time
At specific P-T-X conditions, reactions in solid state toke place in
the
rock
and
new
equilibrated
mineral
assemblage
and
corresponding textures are arise, which equivalent to the
influence metamorphic conditions.
Protoliths of metamorphic rocks
The protoliths of the metamorphic rocks could be:
-Sedimentary rocks
-Shales (Pelitic rocks)
-Sandstones (Arenaceous rocks and semipelitic rocks)
-Carbonate (Calcareous rocks)
-marl rocks
- Igneous rocks
- Basic igneous rocks (metabasites)
- Ultramafic rocks
- Granitoid rocks
- Prior metamorphic rocks